Hydro-electric power plants are built on or adjacent to rivers. The power plants include turbines which are placed in the flow of the river. The moving water causes the turbines to rotate, producing rotational energy, and this energy is then converted to electricity. The power plant turbines typically utilize as much of the flowing water as possible. Often, diversion canals, also referred to as sluice ways, are built along the sides of the power plant to allow debris to be diverted around the turbines. The canals are relatively narrow, minimizing the amount of water diverted from the turbines.
Many, if not most, of the rivers on which hydro-electric power plants are built are inhabited by various varieties of fish. The fish, which regularly swim up and down the river to breed and/or feed, must swim past the power plants. For example, fish living downriver from a power plant may travel upriver to lay eggs. Thereafter the adult fish, as well as the young fish hatching from the eggs, follow their instinctual calling and swim back downriver to feed.
Many fish do not survive the downriver journey because they swim into the power plant turbines and are killed. Observers have found that the fish hesitantly approach the turbine intakes, and after evidently sensing no danger, swim directly into the turbines to their death or fatal injury. A mechanism to alter the behavior of the fish, keeping them from the turbines, without affecting the flow of water is desirable. Such mechanisms may also be used by nuclear power plants or other thermal power plants to deter fish from cooling water intakes.
After much study of fish behavior and particularly fish "sensing" abilities, scientists have found that fish possess highly developed acoustic sensory systems. These systems include tiny "hair cells" which are connected to lateral-line and inner-ear sense organs. The hair cells move in response to the pressure, direction and velocity of sound waves propagating from underwater sound sources and the lateral-line and inner-ear sense organs "interpret" the hair cell movements. For a more detailed discussion of the fish acoustic sensory system, see Kalmijn, Ad.J. (1985) "Hydrodynamic and Acoustic Field Detection" in Sensory Biology of Aquatic Animals, Jelle Atema et. al. (editors), Springer-Verlag, New York, pages 83-129.
Basically, the fish sense the movement of the water associated with the sound waves, sensing what can be characterized as the frequency, amplitude and direction of the sound waves. Certain sounds, that is, certain sound wave patterns, are associated with danger, while other sound wave patterns identify nearby prey or are used for communication. For example, observers have found that the sound waves associated with a series of fish tail-flaps direct a school of fish to make a turn. Thus fish schools have been observed turning in unison following a series of tail-flaps.
Sound generating devices producing loud noises, and corresponding large sound waves, have been used in the past to scare fish away from particular areas. For example, loud sounds have been used to scare sharks from an area making it safe for divers, as shown in Barrand U.S. Pat. 3,317,889. This works well to rid an area of fish for a short period of time. However, if the fish are repeatedly subjected to the same sound, or if the sound is continuous, the fish will sense that there is no danger associated with it. Thus the fish will eventually habituate to the sound and no longer scatter.
Scaring and scattering fish which have strong instinctual yearnings to swim downstream will not work to keep the fish out of the turbines. The fish will eventually sense there is no danger associated with the sound and continue their downstream journey past the sound source and into the turbines. Furthermore, it has been observed that fish typically travel the same routes downstream, for example, swimming with the current or along the shore. Thus using loud noises to simply scatter the fish will not significantly alter the behavior of the fish and the fish will not be kept from the turbines for long. Instead, a mechanism to modify fish behavior, directing the fish to swim around the turbine intakes, is desirable.